Some color laser printers (called a printer hereinafter) 100 employ a printing method called a tandem method that enables the printer to print in parallel a visible image consisting of four colors, Y (yellow), M (magenta), C (cyan) and B (black), as shown in FIG. 1, so that the printer can perform high-speed printing. In order to form a four-colored visible image in parallel, the printer 100 employing the tandem method is provided with four sets of writing systems 110, with each system including an electric discharger 105, a photosensitive drum 106, an electric charger 107, a light source 200, and a developing device 108, as shown in FIG. 2.
A recording paper 120 on a tray 101 as shown in FIG. 1 is fed in a traveling route 103 inside the printer 100 by a carrying roller 102. While the carrying roller 102 is carrying the recording paper 120, writing light emitted from the light source 200 forms a latent image on each photosensitive drum 106 per color, and then the developing device 108 forms a visible image.
The visible image formed on each photosensitive drum 106 is transcribed on the recording paper 120 in the traveling route 103, and then a fixing device 109 fixes the visible image thereon. After that, the recording paper 120 is outputted from the printer 100.
The light source 200 is provided with a substrate 601 extended in a main scanning direction on which light emitting elements 8 consisting of a number of LEDs (Light emitting Diode) are formed, as shown in FIG. 3. Each light emitting element 8 emits a ray of light A in a direction perpendicular to the substrate 601. As shown in FIG. 3, the ray A passes through light transmitting structure 310 such as a rod lens or a fiber lens, which composes the light source 200, and forms the latent image on the photosensitive drum 106.
The light transmitting structure 310 has a narrow angular aperture and a depth of focus is maintained to be long so that a clear latent image may be formed on the photosensitive drum in a simple manner.